FIG. 1a shows a known embodiment of a precipitator designed from cellulose material, said precipitator including two electrode elements 1, 2 arranged with a mutual gap width “d” and arranged in planes parallel to each other. As is evident from FIG. 1b the electrode elements 1, 2 are electrically connected to respective poles of a high voltage source HVU through galvanic connection to an electrically semi-conducting or current carrying wire drawing a, b attached to the edge portions k1, k2 of the respective electrode elements 1, 2.
The circumstances concerning voltage-current that is valid between the electrode elements 1, 2 are shown in FIG. 1b. One pole of the high voltage source HVU is electrically earthed and is connected to the current carrying edge portion k1 of one electrode element 1. The other alive pole (+) is connected to the current carrying edge portion k2 of the other electrode element 2 (wire drawing b). In this case the edge portion and the wire drawing coincide. The width of the electrode elements 1, 2, seen in the air flow direction through the precipitator, is equal to “B”. The voltage across the gap between the adjacent edge portions k1-k2′, k1′-k2 is designated Uk and corresponds to the voltage that maintains the corona discharge current Ic from the edge portions k2, k2′.
At the top of FIG. 1c a voltage diagram is drawn for the electrode element 2 as a function of the width “B” of the electrode element 2. The diagram over the electrode element 2 shows that there is a linear increase in voltage from the voltage level Uk, closest to the edge portion k2′, to the corresponding U′=HVU(+) at the edge portion k2, i.e. the alive pole of the high voltage source having the highest potential.
The intermediate diagram in FIG. 1c shows the corresponding voltage diagram for the electrode element 1 where the voltage is equal to zero at the edge portion k1, said voltage increasing linearly to the voltage level U″=HVU(+)−Uk at the edge portion k1′.
By positioning both diagrams in one, at the bottom of FIG. 1c, the gap voltage Usp is given as a function of the width “B” of the electrode elements 1, 2.
For reasons of simplicity the corona current from the edge portions n′-m′, m-n has been disregarded. For band like electrode elements having a length “L” that is several times the width this assumption is perfectly correct. For rectangular electrode elements the approximation is acceptable under the prerequisite that the width of the electrode elements is considerably larger than their extension in the direction of the air flow or that the edge portions n′-m′, m-n are included, e.g. by use of electrically insulating material.
As FIG. 1c shows the gap voltage Usp between two electrode elements 1, 2 of very high ohmic material is essentially constant over the entire gap and the width “B” of the electrode elements, seen in the direction of the air flow, and equal to the voltage Uk that upholds the corona discharge current Ic.
If the diagram shown in FIG. 1d is considered, said diagram showing approximately the corona discharge current Ic as a function of the voltage Uk between edge portions of two adjacent electrode elements, it is realised that the steeper the curve is, i.e. the larger the derivative (Ic1-Ic2)/(Uk1-Uk2) is, the less the level of the gap voltage Usp is affected by increasing high voltage supply HVU. In other words the gap voltage Usp between two electrode elements designed of very high resistive, preferably antistatic, material (inside the voltage area above the treshold value for corona discharge between the edge portions of the electrodes) is only to a minor degree affected by increasing supply voltage (high voltage HVU) to those electrode elements.
By increasing air humidity (Rh—relative air humidity), i.e. Rh1>Rh2 a displacement towards lower voltage levels of the threshold voltage of edge corona discharge takes place, this being verified in the laboratory tests (see FIG. 1e). Simultaneously the derivative increases (Ic1-Ic2/Uk1-Uk2), i.e. the edge corona voltage as a function of the edge corona current increases towards a steeper progress. Thereby, a considerable decrease of the edge corona voltage Uk and hence a decrease of the gap voltage Usp takes place by increasing air humidity and at a constant edge corona current (Ic=constant). The ability of high resistive precipitators to separate particles decreases to the same extension. The understanding as outlined above constitutes the base of the present invention.